Reduced turbulent transport in quasi-helically symmetric stellarators

A distinctive feature of quasi-helically symmetric (QHS) stellarators is their relatively small connection length. This has several beneficial MHD consequences including reduced Pfirsh-Schlüter and bootstrap currents and smaller Shafranov shifts relative to equivalent scale tokamaks at comparable plasma beta. In this work, we argue that the shorter connection lengths of QHS will also lead to superior turbulent transport properties. While QHS stellarators tend to have larger normalized growth-rates relative to other configurations due to reduced connection length, nonlinear gyrokinetic simulations show that QHS stellarators have net lower turbulent heat transport than other optimized stellarator options[1]. We argue the appropriate spatial scale for ITG-like turbulence is set by the banana width. Following this logic for conventional stellarators, one can produce stellarator energy confinement time scaling laws that are remarkably consistent with the empirically derived ISS04 scaling. The reduced banana widths present in QHS stellarators imply that the reduced connection length has a net beneficial effect on turbulent transport. Nonlinear gyrokinetic simulations are performed in support of this premise.



[1] I. J. McKinney et al, J. Plasma Physics 85, 905850503 (2019).